The cooling of a mold and a metal product to reduce a surface temperature and/or to obtain the uniform temperature distribution at the surface (or volume) of the mold and the metal product is very important in determining their performance and life time in many cases.
For example, the cooling of injection mold is crucial to the performance of the mold influencing both the rate of the process and the resulting quality of the products produced. In plastic injection molding, the high surface temperature of the mold extends the cycle time to manufacture a plastic product, and the non-uniform temperature distribution at the surface of the mold induces various defects in products such as warpage, thickness variation, and/or a large amount of micro-pores. Meanwhile, the cooling of the injection mold reduces the surface temperature of the mold and uniformalizes the temperature distribution at the surface of the mold, whereby the cooling time and the cycle time required for manufacturing the plastic product is shortened, resulting in an increase in the productivity, as well as improvements in the quality of the plastic products.
However, cooling channel (line) fabrication has been confined to relatively simple configurations, such as straight cooling channel fabricated by gun-drilling, baffle cooling channels, and the like, primarily due to the limits of the conventional metal fabrication methods (including cutting, casting, electrical discharge machining, etc.) used to manufacture the mold and the industrial metal product. Practically, the mold and the metal product with complicated and/or curved cooling channel cannot be made by using the conventional metal fabrication methods because freely manufacturing the configuration and the geometry of the cooling channel is impossible by using the only combination of the straight and the baffle cooling channel made by the conventional metal fabrication methods.
In addition, since the fabrication of the straight cooling channel and the baffle cooling channel does not allow for the formation of a curved channel in which the direction of a coolant is smoothly altered at a turning portion, turbulence or congestion is generated in the coolant flow at a drilling-overlap portion and/or a portion provided with a baffle to thereby lead to extremely low cooling efficiency.
Recently, the emergence of additive fabrication (AF) technologies able to make geometrically complex metal parts and molds directly from 3-dimensional (3D) CAD data has made it possible to produce metal parts and molds with conformal cooling channels that cannot be manufactured by conventional metal fabrication methods.
The basic concept of building a 3D object directly from 3D CAD data in the AF technologies is illustrated in FIG. 1. Referring to FIG. 1, the 3D CAD data is divided to get a set of thin layer data (2D cross-sectional information) with a predetermined thickness (or height) and then each successive metal layer corresponding to the 2D cross-sectional information is formed upon a previously formed layer in sequence by using a method of sintering or melting a powdered metal (FIGS. 1A to 1C), producing a 3D metal part (FIG. 1D).
In most of AF technologies, a metal powder is used. According to a method of supplying metal powder in fabricating a 3D metal part, AF technologies can be typically classified into 1), a powder pre-placement method in which metal powder is initially spread at a predetermined thickness before a sintering or melting process is preformed thereupon and 2), an in-situ powder-feeding method in which metal powder is supplied in real time during a build-up process.
The powder pre-placement method 1), among AF technologies, includes a selective laser sintering (SLS) technique and a selective laser melting (SLM) technique (hereinafter, SLS and SLM techniques will be explained as including techniques, such as trademarked DMLS (direct metal laser sintering), LaserCUSING, EBM (electron beam manufacturing), or the like, in this disclosure).
In the SLS and the SLM techniques, metal powder is precisely spread at a predetermined, constant thickness and then a laser beam or electron beam is selectively irradiated onto the layer of metal powder to locally sinter or melt the metal powder, thereby fabricating a two-dimensional metal layer. Then, a series of processes of spreading metal powder at a constant thickness and sintering or melting the metal powder are repeatedly undertaken, upon one another, to thereby manufacture a metal product having a 3D shape.
In particular, the SLS and the SLM techniques are advantageous in terms of the manufacturing of an overhang structure having an empty space therebelow, because metal powder provided below, not irradiated by a laser beam or electron beam, acts as a kind of support in the processes, such that the SLS and the SLM techniques may be theoretically suitable for forming the structure of a cooling channel.
However, the SLS and the SLM techniques have limitations in that a great deal of mold manufacturing costs is incurred due to the use of a relatively expensive special metal powder, water leakage may occur in a manufactured cooling channel due to defects, such as cracks, pores, or the like, a corrosion rate is relatively rapid due to the rough surface of the manufactured cooling channel, a clogging phenomenon has been often reported in the cooling channel, the size of a manufactured product is limited, or the like. Thus, these techniques have not been widely industrially employed in manufacturing the three-dimensional cooling channel in practice.
Meanwhile, the in-situ powder-feeding method 2), of supplying metal powder in real time, among AF technologies, includes direct metal fabrication (DMF) and multilayer laser cladding techniques, and the like.
As illustrated in FIG. 2, these techniques allow for the formation of a metal product having a 3D shape on the base B by using a forming device M including a laser beam irradiator and a powder feeder.
In the DMF and multilayer laser cladding techniques, a high-powered laser beam is irradiated onto a metal surface of a work piece to form a molten pool of metal into which a precise amount of metal powder is injected in real time. At the same time, by moving the laser beam or (and) work piece along a tool path calculated from 3D CAD data, a metal layer corresponding to a portion of 2D cross-sectional data is formed. Such a process is repeated in sequence, layer-by-layer to thereby manufacture a metal product identical to a 3D CAD model.
The DMF and multilayer laser cladding techniques use commercial metal powders in industry, and result in a fully dense metal product having a fine microstructure due to complete melting and rapid solidification during processing. The metal parts and molds fabricated by the DMF and multilayer laser cladding techniques show excellent mechanical properties equivalent or superior to wrought (or forged) metal in many cases.
In particular, because of supplying the metal powder in real time during the process, the DMF and multilayer laser cladding techniques can form a metal part or 3-dimensional geometry on a 3-dimensional curved surface of metal work piece (or metal substrate), as well as on a 2D flat surface of metal work piece (or metal substrate), unlike the SLS and the SLM techniques can only perform a process on a 2D flat surface.
However, since the DMF and multilayer laser cladding techniques do not include metal powder (or a metal powder layer) acting as a support, unlike the SLS or SLM technique, the manufacturing of an overhang structure having an empty space formed in the lower side thereof is not facilitated.
Accordingly, in order to form the overhang structure (including a cooling channel having the overhang structure) through the DMF and multilayer laser cladding techniques, controlling complex motions of relatively expensive 5-axis equipment is generally required.
There are also alternative methods to allow for producing the overhang structures without the complicated 5-axis motion in the DMF and multilayer laser cladding techniques, such as fabricating a support structure in the process by using an additional metal powder having a low melting point or inserting a flexible copper tube.
In the case of using the additional metal powder having a low melting point to fill an internal empty space of a cooling channel with a support structure or to make a support structure, after a final metal product is fabricated, heating process is necessary for removal of the metal with a low melting point. In this case, there are limitations in that an additional process for manufacturing the cooling channel is required, the surface of the cooling channel can be rather rough, and corrosion (in particular, galvanic corrosion) in the cooling channel can occurs due to metal having a low melting point remaining in the cooling channel without being completely removed.
In the method of inserting the flexible copper tube in order to form the cooling channel, as illustrated in FIGS. 3A through 3D, a primary metal product 2 is first fabricated using the DMF or multilayer laser cladding techniques such that a mounting groove 3 is provided in a base 1 (FIG. 3A), and then the copper tube 4 is inserted into the mounting groove 3 (FIG. 3B). By continuously using the DMF or multilayer laser cladding techniques, metal layers 5 are repeatedly formed on the copper tube 4 (FIG. 3C), and then a final metal product having the copper tube 4 inserted thereinto is formed (FIG. 3D).
The method of inserting the copper tube illustrated in FIGS. 3A through 3D has advantages, such as simplicity of manufacturing, smooth surfaces, and high corrosion resistance in the cooling channel; however, it has also disadvantages, such as a reduction in cooling efficiency, because an upper portion of the inserted copper tube 4 forms a complete metallic bond with the metal layers 5 (or the metal product) deposited on it, while a lower portion of the copper tube 4 installed in the mounting groove 3 is separated from the metal product and is not bonded thereto.
In particular, in the case of sharply bending the copper tube 4 in order to form a complex and curved cooling channel, uneven cross-sections in bent portions of the copper tube 4 can be formed, resulting in the occurrence of turbulence in a coolant flow within the copper tube in the event of excessive bending of the copper tube.
Further, since the mounting groove 3 has a cross section corresponding to an only half circle of the copper tube 4, the copper tube 4 is not completely in contact with the cross section of the mounting groove 3 and tends to lift from the mounting groove 3. When the cooling channel has a complex flexion as illustrated in FIGS. 3A through 3D, the magnitude of this phenomenon is increased. Further, when the copper tube 4 is installed along a 3D cooling channel, the lifting phenomenon becomes evident.
In this manner, in the case of the occurrence of the lifting phenomenon, there are many difficulties in manufacturing an upper portion of a final metal product on the copper tube 4 through the DMF and multilayer laser cladding techniques. That is, while the DMF and multilayer laser cladding techniques allow for the deposition of a metal layer corresponding to a 2D cross-section along a path calculated from 3D CAD data, the configuration of the path calculated from 3D CAD data and the actual configuration of the copper tube 4 are different at the location of the occurrence of the lifting phenomenon to thereby cause difficulties in the attainment of a perfect molding.
Ultimately, a method of forming a 3D cooling channel (or an internal space), different from the method of forming a 3D cooling channel by using the AF technologies according to the related art, is required.